Acoustic walls and barriers block sound waves from traversing from one side to the other. Acoustic walls can block sound waves by various mechanisms. First, by providing a massive wall the sound can be reflected or scattered to prevent it from traversing to the other side. Second, a wall can be made to have particular vibration and resonance properties to attenuate the sound waves by absorbing them.
In the field, acoustic panels have been constructed using various types of materials. For instance, one type of acoustic panel has used gypsum-steel faces, wherein each face is composed of a gypsum board and a steel sheet secured to each other. The faces are mounted in spaced relation to each other in a frame and often rock wool is provided in the gap between the spaced faces. Such acoustic panels often have an acoustic performance of about 53 STC. The increasing cost of primary materials, particularly steel, has rendered this known configuration relatively expensive.
Some known acoustic articles have been composed of a matrix material with small embedded particles to provide acoustic properties. Matrix materials have been cementitious materials such as concrete or polymers. The particles incorporated in the matrix have been to a large extent in powder form and have been made of materials such as lead, barium, iron, glass, silicone or sand.
Some powder-containing compositions have been used to provide lightweight acoustic articles that can absorb certain frequencies of structural vibration or sound waves. Most powders or particles have been used in relatively low weight percentage in relation to the weight of the overall composition.
Incorporating powders or fine particulate materials into a matrix has various disadvantages. For instance, fine material may tend to clump or stick to vessels during handling and manufacturing, which reduces efficiency and makes it more difficult to consistently and evenly distribute the material in the matrix. This can result in reduced reliability in the final product. It may also be difficult to incorporate a high mass percentage of fine material consistently into the matrix, particularly when even distribution is desired.
Principles of Acoustic Transmission Loss
Transmission loss is an acoustic indicator and is governed by certain principles particularly in relation to single- and multiple-face configurations.
Single Panels
The transmission loss of a single face is characterized by three zones:
1) Mass law. At low frequencies the transmission loss of a single face is governed by the mass law, which can be represented by the following formula:
  R  =      20    ⁢                  ⁢          log      ⁡              (                              m            ⁢                                                  ⁢            ω                                2            ⁢                          ρ              0                        ⁢            c                          )            
where R is the attenuation,                m is the surface density of the panel,        ω=2·π·f is the frequency, ρ0 is the density of the air and        c is the wave velocity of the sound in the air.This relationship indicates that by doubling the surface density leads to an increase in the transmission loss of 6 dB. The transmission loss increases linearly with the frequency according to a slope of 6 dB per octave.        
2) Critical frequency. The critical frequency is characterized by a marked decrease in the acoustic efficiency of the face. At this frequency, the wave velocity in the face is equal to the wave velocity in the surrounding fluid medium. This phenomenon optimizes the acoustic energy transfer into vibrational energy and therefore decreases the efficiency of the face. The value of the critical frequency depends on the deflection rigidity of the panel and the propagation conditions in the surrounding fluid. In practice, the critical frequency is calculated with the following formula:
      f    c    =                              c          2                          2          ⁢          π                    ⁢                        m          D                      =                            c          2                          2          ⁢          π                    ⁢                                    12            ⁢                                                  ⁢                                          ρ                s                            ⁡                              (                                  1                  -                                      v                    2                                                  )                                                          Eh            2                              
where                fc, is the critical frequency;        c, is the wave velocity in the fluid environment;        m, is the surface density;        D, is the deflection rigidity of the material;        ρs, is the surface density of the face;        v, is the Poisson coefficient of the face;        E, is the Young's modulus of the face;        h, is the thickness of the face.        
3) Stiffness-governed transparency. At high frequencies, above the critical frequency, the transparency of the face depends principally on its stiffness. Increasing the transmission loss is achieved at a slope of 12 dB per octave.
Double-Panel Configuration
The transmission loss of a double-panel is characterized by three different zones.
1) At low frequencies, the transmission loss presents a singularity with a significant drop in value. This singularity may be called the “respiration frequency” of the double-face and is evaluated by the following formula:
      f    dp    =            1              2        ⁢                                  ⁢        π              ⁢                                        ρ            ·                          c              2                                d                ⁢                  (                                    1                              m                1                                      +                          1                              m                2                                              )                    
where                fdp, is the respiration frequency of the double-face;        ρ, is the fluid density;        c, is the wave velocity in the fluid environment;        mi, is the surface density of the first (1) or second (2) face;        d, is the distance between the faces.        
2) At medium frequencies, the transmission loss of the double-face is governed by the acoustic resonance in the cavity between the two faces. These resonances can be eliminated by employing an absorbent material in the cavity.
3) At high frequencies, the phenomena of critical frequencies arise for each of the two faces.
Known acoustic faces and panel assemblies present a variety of disadvantages such as using expensive materials, being relatively light which hampers their sound blocking ability, being difficult or inefficient to manufacture and/or containing compounds that may be toxic or present other drawbacks.
There is a need in the field of acoustics for a technology that overcomes at least one of the disadvantages of known acoustic faces and panel assemblies.